Various agricultural, construction and other industrial equipment use hydraulic cylinders to control the movement and position of the machinery. In general, the cylinder assembly has a cylinder body with a cylinder rod extending therein. The cylinder rod may be connected or secured to a piston at the one end and coupled (directly or indirectly) to a machine component at the end that extends out of the cylinder body. Fluid enters the cylinder body, causing the piston and the cylinder rod, which is secured thereto, to move relative to the cylinder body. The movement of the cylinder rod drives the motion of the machine component.
Precise control of the position of the piston is important to controlling the operation of the machinery. Measuring the absolute position or velocity of the piston relative to the cylinder is often needed to achieve the desired control.
U.S. Patent Application Publication Number 2004/0222788 describes a system and method of recording piston rod position information in a magnetic layer on the piston rod. A piston rod moving with respect to a cylinder has a magnetically hard layer formed thereon to provide a recording medium. A magnetic pattern is recorded in the magnetically hard layer. A magnetic field sensor senses the recorded magnetic pattern while the piston rod is moving with respect to the cylinder and generates signals in response to the magnetic pattern that are used to determine an instantaneous position of the piston rod. This is a relatively complicated and costly device. The magnetic pattern only allows the magnetic field sensor to sense the relative position of the piston rod, not the absolute position.
U.S. Pat. No. 7,051,639 discloses a method and apparatus for detecting the position of a rod member of a cylinder assembly. The cylinder assembly has a cylinder body with a cylinder chamber therein, a gland member disposed within the cylinder chamber, and a rod member movably arranged within the cylinder chamber and a rod opening formed in the gland member. The method includes moving the gland member within the cylinder chamber to substantially align a gland aperture of the gland member with a cylinder aperture of the cylinder body; substantially fixing the gland member relative to the cylinder body; positioning a sensor with at least one of the cylinder aperture and the gland aperture; moving the rod member within the rod opening of the gland member and the cylinder chamber of the cylinder body; and operating the position sensor to detect the position of the rod member.
U.S. Pat. No. 7,162,947 discloses a cylinder body having a first mounting portion disposed thereon. The gland member may be disposed within a gland opening formed in the cylinder body. The sensor mount has a second mounting portion disposed thereon and may be attached to the cylinder body via a coupling engagement between the second mounting portion of the sensor mount and the first mounting portion of the cylinder body. The rod member may be slidably arranged within rod openings of the sensor mount and the gland member and may extend into a longitudinal cylinder chamber of the cylinder body.
Both of these patents require the cylinder body to be modified to include the sensor. U.S. Pat. No. 7,162,947 requires that a mounting portion be provided on the cylinder body and U.S. Pat. No. 7,051,639 requires that an aperture be provided in the cylinder body. It would be advantageous to provide a sensor which could be retrofitted for use with existing cylinder bodies, without requiring modifications to the cylinder body and/or the cylinder gland.